Process for making polarized material

ABSTRACT

This invention relates to a process for making polarized material by forming a solution of a material capable of being polarized with a polarization solvent which can be removed by evaporation. The material in solution is poled to provide a polarized material which is free or substantially free of mechanically-induced orientation and which polarization is essentially stable up to the crystal melting point of the polar crystals or to softening point of the polarized material if non-crystalline. The polarization solvent can be removed from the solution as desired either to a reduced level or completely, such as during poling or before or after poling.

This invention was made with Government support under the Office ofNaval Research, Grant No. N00014-80C 0795, and the Government hascertain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.881,828 filed July 3, 1986.

TECHNICAL FIELD

This invention relates to a process for making polarized material with anet polarization. It provides an improved and more efficient process formaking such materials and is particularly useful for making polymericmaterials or material products with piezoelectric and pyroelectricproperties. Also, by this invention is provided a polarized materialwhich is essentially stable up to its crystal melting point or itssoftening point, if the polarized material is non crystalline. Thepolarized material is substantially free of mechanically-inducedorientation and has mechanical and electromechanical propertiesisotropic in a plane perpendicular to the poling field direction.

BACKGROUND ART

Certain materials such as various polymeric materials are capable ofbeing polarized when subjected to mechanical or electrical stresses. Inthe past, it has been found that a polymeric material such aspoly(vinylidene fluoride) can be polarized by stretching a sheet at atemperature of about 70° C. at least three times its length, andsubjecting the stretched sheet to a DC field of at least 1 MV/cm.Poly(vinylidene fluoride) has been a preferred material forpolarization, since it has been found to have a high capability ofpolarization response, thereby providing high piezoelectric orpyroelectric properties or highly desired optical properties. Subjectingsuch a stretched film, for example, using an appropriate DC fieldapplied in a direction perpendicular to the plane of the stretched filmcauses an orientation of the molecular dipoles of the materials. In thecase of poly(vinylidene fluoride), the fluoro groups have a negativecharge and the hydrogen atoms attached to the other carbon of thevinylidene fluoride unit of the polymer have a positive charge.Vinylidene fluoride units in a poly(vinylidene fluoride) can exist in atleast two different crystalline forms. In one form, the vinylidenefluoride units exist in a planar zigzag polar form or trans form (betaform or Form 1). In another form, the form is nonpolar and nonplanar; itis a T-G-T-G' form (alpha form or Form 2) wherein T denotes transconfiguration and G and G' denote the two types of gauche forms. In thepast, the desired increase in Form 1 has been realized by subjectingpoly(vinylidene fluoride) films (PVF₂ films) to stretching andsubsequently subjecting the stretched films to high DC fields overextended periods of time at high temperatures. Such treatment with a DCfield is referred to as "poling". It is desired to have a high contentof Form 1 in order to have the highest amount of desired polarizationproperties, for example, piezoelectric and pyroelectric properties. Thepolarized material is cooled after poling for purposes of retaining thepolarization.

Such polarized materials are used, for example, in making transducers,which utilize the piezoelectric or pyroelectric or other polarizationproperties of such polarized materials.

Various other polarizable polymers having various groups such as fluoro,chloro, amide, ester, cyanide, carbonate, nitrile, ether, and the like,such as polyvinylchloride (PVC), polyvinylfluoride (PVF), vinylidenefluoride copolymers, and many other polymer materials have thecapability of being polarized as do various non-polymeric materials suchas some ceramic materials.

Customary stretching in the film direction causes an unequal (oranisotropic) elastic modules in the stretching or axial direction(X--X¹) as compared to the transverse direction (y--y¹). This isundesirable. It is desired to provide materials which are free orsubstantially free of such mechanically induced orientation and whichhave a polarization which is stable up to the crystal melting point ormaterial softening point, in the case of non-crystalline polarizedmaterials. Such materials are substantially free of said anisotropicmechanical properties. Such polarized materials and processes forproducing such polarized materials are highly desired.

SUMMARY OF INVENTION

A process has been found by which highly polarized materials can beproduced which are free or substantially free of mechanically inducedorientation and which polarization is essentially stable up to about thecrystal melting point (glass transition temperature) of the polarizedmaterial or up to about the softening point of the polarized material inthe case of non-crystal line polarized material. The process comprisesdissolving a material to be polarized in a solvent or solvents for thatmaterial. The solvent is selected which is adapted to the polarizationof the material and which can be removed to the extent desired byevaporation during the course of the polarization or prior to orsubsequent to the polarization. The temperature employed will be one atwhich polarization effectively occurs, ordinarily at an elevatedtemperature below which substantial dielectric breakdown occurs. The DCfield employed in the polarization will be selected which provides thedesired polarization.

Also provided by this invention are polarized products which are free orsubstantially free of mechanically induced orientation and which areessentially stable up to about the crystal melting point of the materialor in the case of noncrystalline material up to about the softeningpoint of the material.

The material presently preferred is poly(vinylidene fluoride) or certaincopolymers of vinylidene fluoride.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of an apparatus used to provide theDC field for the polarization of the material and to provide as desiredfor removal of solvent by evaporation during poling.

FIG. 2 is a graph showing the DC field, the content of solvent presentand the temperature of polarization in illustration of the process inwhich poly(vinylidene fluoride) is used as the polarized material.

FIG. 3 is a schematic representation of an apparatus for carrying outthe invention by which the electric field is provided by coronadischarge and the solvent can be removed by passing a gas over thesurface of the solution of the material to reduce solvent content asdesired.

FIG. 4 is a perspective view of a transducer using polarizationpoly(vinylidene fluoride) film made by the process of this invention asthe active piezoelectric material.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

The process is carried out by first dissolving the material to bepolarized in the required amount of a suitable solvent or solvents toform a solution. For example, if poly(vinylidene fluoride) is selectedas the material, a suitable solvent such as tricresylphosphate can beused. It has been found that about five parts of a poly(vinylidenefluoride), which is suitable for making a polarized film, is anacceptable amount to dissolve in 95 parts of tricresylphosphate oranother polarization solvent for making the solution for polarization.The mixture is heated to about 180°-190° C. to aid dissolution. It hasbeen found that a capacitor grade poly(vinylidene fluoride) as sold byKureha Kagoku Kogko Kabishiki Kaisha is suitable.

Ordinarily, it is preferred to reduce the solvent content in thesolution prior to commencement of polarization. For example, in the caseof poly(vinylidene fluoride)/tricresylphosphate solution, the solventcontent can be reduced from 95 parts to 50 parts or below such as to26.5 parts as shown in FIG. 2 or lower providing the poly(vinylidenefluoride) remains in solution.

The solution with the reduced solvent content then is preferably placedin a press under a suitable pressure of 2500 to 3500 psi at an elevatedtemperature below the degradation temperature of the polymer; forexample, it has been found suitable to use a pressure of 3000 psi and atemperature of 185° C. The film then is preferably cooled rapidly a byimmersion in an ice bath.

As shown in FIG. 1, the solution of poly(vinylidene fluoride) with thereduced solvent content can be placed into a suitable vacuum oven whichis equipped with an appropriate DC source. The poly(vinylidene fluoride)solution is placed between two suitable electrodes. Polished copperplates can be used as the electrodes. The electric field is provided.Temperature is adjusted as desired. A temperature can be used which willprovide a high rate of removal of the solvent and a high rate ofpolarization without substantial dielectric breakdown of the materialbeing polarized. The solvent content can be reduced if desired byevaporation prior to poling and can be further reduced after poling, ifdesired.

The process can be carried out to provide a desired polarizedpoly(vinylidene fluoride) film as shown in FIG. 2.

The solvent can be reduced in the polarized material as desired.However, it has been found that retaining a small amount of solvent inthe polarized material provides desirable improvement in the dielectricconstant. For example, the dielectric constant of the poly(vinylidenefluoride) film can thereby be increased by up to about 400 percent,preferably from about 50 percent to about 300 percent and still retainan essentially stable polarization up to the crystal melting point or tothe softening point of the material used in the polarization process, asthe case might be.

The poling can also be carried out using corona discharge to provide theelectrical field. As shown in FIG. 3, a conventional corona dischargeapparatus can be used. The grid as shown in FIG. 3 is placed above thesolution of material to be polarized. As shown in FIG. 3, the solventcontent can be reduced during the polarization by passing a flow of asuitable gas over the surface of the film. It has been found thatnitrogen ordinarily is a suitable gas for this purpose. The gas with theuptaken solvent can be processed in a conventional manner to remove orreduce the content of solvent and the gas can be passed in a continuousmanner again over the film being polarized. If desired, the solventcontent of the solution can be reduced prior to poling and then can befurther reduced after poling if desired.

Also, the process can be made continuous by placing the solution film ona moving conveyer in suitable arrangement of corona discharge elementstogether with an appropriate means of solvent removal, as a passage of asuitable gas over the surface of the material solution, if solventremoval is desired during poling.

The intensity of the electric field used will ordinarily be selected toprovide efficient polarization. However, it will be kept below the rangeat which substantial dielectric breakdown of the material beingpolarized occurs. In the case of poly(vinylidene fluoride), it has beenfound that an electrical field of 250 KV/cm is satisfactory to pole apoly(vinylidene fluoride) film solution having a 23.5 percent solventcontent, as shown in FIG. 3. Also, it is shown in FIG. 3 that theelectrical field could satisfactorily be increased to 500 KV/cm as thesolvent content decreased and increased further to 750 KV/cm when thesol vent content is reduced to about 16 percent. The initial electricalfield could have been reduced below 250 KV/cm such as to 50-200 KV/cm;however, using lower electrical fields than those in a maximum rangeresults in less efficient polarization. Under the conditions of FIG. 3,the further increase of the electrical field beyond 750 KV/cm at 16percent solvent can result in some dielectric breakdown, for example, inthe range of about 800 to about 1000 KV/cm. Care should be exercised tostay below an electrical field of such intensity that dielectricbreakdown of the material occurs. The electrical field at whichsubstantial breakdown occurs can be determined by preliminaryexperimentation.

The temperature at which the polarization process is carried out dependsupon the desired rate at which polarization occurs, the material used,the solvent used, the equipment available for polarization, the desiredlevel of solvent wished to be retained in the final polarizationmaterial and other factors. As is shown in FIG. 3, a starting polingtemperature of 45° C. is used satisfactorily. The temperature isincreased to 60° C. and finally the temperature of 90° C. is used whensolvent content is reduced to about 16 percent. Obviously, the polingtemperature should be maintained lower than the boiling point of thesolvent under the conditions used.

Materials which can be used in this invention can vary widely so long asthey have a polarization capability. As mentioned above, a preferredmaterial is poly(vinylidene fluoride). Copolymers of vinylidene fluorideare also desirable materials, such as vinylidene fluoride copolymerswith vinyl fluoride, trifluoroethylene, tetrafluoroethylene, vinylchloride, methylmethacrylate, and others. The vinylidene fluoridecontent can vary in the range of from about 30 percent to about 95percent based on the total polymer weight. Other polymers which can beused are polyvinylchloride, polymethylacrylate, polymethylmethacrylate,vinylidene cyanide/vinyl acetate copolymers, vinylidene cyanide/vinylbenzoate copolymers, vinylidene cyanide/isobutylene copolymers,vinylidene cyanide/methyl methacrylate copolymers, polyvinylfluoride,polyacrylonitrile, polycarbonate, and nylons such as Nylon-7 andNylon-11, natural polymers such as cellulose and proteins, syntheticpolymers such as derivatives of cellulose, such as esters and ethers,poly (-methyl-L-glutamate), and the like. Also, polarizable materialswhich are soluble ceramic materials and capable of forming polarcrystals or glasses can be used together with an appropriatepolarization solvent for particular soluble ceramic material used.

A variety of suitable solvents can be used depending upon the materialused in the polarization, cost and safety consideration, equipment used,and other factors. In the use of poly(vinylidene fluoride) material,tricresylphosphate has been found to be a suitable solvent. It is alsosuitable for use when many copolymers of vinylidene fluoride are used.Dibutyl phthalate can also be used as the solvent for these vinylidenepolymers. In the use of nylon-7 and nylon-11, 2-ethyl-1,3-hexanediol canbe used. Other solvents can be used depending upon the polymer materialused and other factors and will be suggested to those skilled in theart.

The term solution as used herein has its usual meaning of a mixture oftwo or more elements or compounds which appear to be homogeneous even tothe highest possible magnification of visible light. The Encyclopedia ofChemistry, 2nd ed., Ed. George L. Clark, Reinhold PublishingCorporation, New York, N.Y., 1966, page 989.

Polarized materials provided by this invention can be used in makingtransducers having standard transducer configurations. FIG. 4 shows aperspective view of a transducer 40 using polarized poly(vinylidenefluoride) film as the active piezoelectric material. Transducer 40includes a long and flat outer layer 42 and a piezoelectric cable 44 isenveloped in outer layer 42. Piezoelectric cable 44 is provided with apiezoelectric layer or sheet 46 of poly(vinylidene fluoride) rolled onthe outer periphery of a center electrode 50 which is made of a thinmetallic wire. The outer electrode 52 made of a thin metallic film isformed on the outer periphery of layer 46. In operation, when pressureinduced by an acoustic signal is exerted on the outer layer 42, it istransmitted to layer 46 of poly(vinylidene fluoride) and electricpotential is generated between electrodes 50 and 52. Conversely, apressure wave is generated by the transducer when an electrical signalis applied between the two electrodes 50 and 52.

Measurements of piezoelectric strain constant, d₃₁, piezoelectric stressconstant, e31, pyroelectric constant, p_(y), dielectric constant anddynamic mechanical modulus were determined in conventional manner(measured at 3 Hz). The percentages of crystallinity is determined byX-ray diffractometer scans.

Example 1

Five parts of Kureha capacitor grade poly(vinylidene fluoride) (PVF₂)film are dissolved in 95 parts of tricresylphosphate at 185° C. Thesolution is transferred to a tray and placed into a vacuum oven. Theoven is maintained at a vacuum of about 10⁻³ torr and at a temperaturewithin the range of 150° C.-200° C. until a PVF₂ solution is obtainedhaving about 70 percent by weight of PVF₂ and 30 percent by weight oftricresylphosphate. The PVF₂ solution is transferred as a film to apress, subjected to a pressure of 3000 psi and heated to 185° C. Thefilm is then rapidly cooled by immersion into an ice bath. The PVF₂solution film now comprises 76.5 percent by weight of PVF₂ and 23.5percent by weight of tricresylphosphate. This film having a thickness ofabout 1 mil is transferred to a poling apparatus of the type depicted inFIG. 1 between two polished copper plates, is connected to a highvoltage DC supply, and then is placed under high vacuum (˜10⁻⁶ torr).The PVF₂ solution film is poled as shown in FIG. 2 at 250 KV/cm at 45°C. for about 45 minutes, at which point the field is linearly increasedto 500 KV/cm at poling time of about 2.3 hours, while maintaining thetemperature at 45° C. The temperature is then increased to 60° C. Atpoling time of 3 hours, the field is linearly increased to 750 KV/cm atpoling time of 4.8 hours, at which time the temperature is increased to90° C. Poling is continued at 750 KV/cm field for about 30 minutes atwhich time the temperature is permitted to return to room temperature.When room temperature is reached, the field is reduced to zero. Retainedsolvent content is about 17 percent.

The polarized material shows the following properties:

    ______________________________________                                        d.sub.31          13 pC/N                                                     d.sub.33          32 pC/N                                                     e.sub.31          9.5 mc/m.sup.2                                              p.sub.y           22.5 uC/m.sup.2 K                                           dielectric constant                                                                             31                                                          dynamic mechanical modules                                                                      0.6 × 10.sup.10 dynes/cm.sup.2 (at 3 Hz)              dp                5 pC/N                                                      ______________________________________                                    

Example 2

Five parts of Kynar copolymer VF₂ VF₃ (80% VF₂) film produced byPennwalt Corporation are dissolved in 95 parts of tricresylphosphate at240° C. The solution is transferred to a tray and placed into a vacuumoven. The oven is maintained at a vacuum of about 10⁻³ torr and at atemperature within the range of 100° C.-120° C. until a copolymersolution is obtained having about 70 percent by weight of copolymer and30 percent by weight of tricresylphosphate. The copolymer solution istransferred as a film to a press, subjected to a pressure of 3000 psiand heated to 125° C. The film is then rapidly cooled by immersion intoan ice bath. This film having a thickness of about 1 mil is transferredto a poling apparatus of the type depicted in FIG. 1 between twopolished copper plates, is connected to a high voltage DC supply, andthen is placed under high vacuum (˜10⁻⁶ torr) The copolymer solutionfilm is poled generally in a manner as shown in FIG. 2 at 250 KV/cm at45° C. for about 45 minutes, at which point the field is linearlyincreased to 600 KV/cm at poling time of about 4 hours while maintainingthe temperature at 45° C. The temperature is then increased to 60° C.Poling is continued at 600 KV/cm field for about 15 minutes at whichtime the temperature is permitted to return to room temperature. Whenroom temperature is reached, the field is reduced to zero.

The polarized material shows the following properties:

    ______________________________________                                        d.sub.31          9.66      pC/N                                              e.sub.31          2.3       mc/m.sup.2                                        modulus           0.18 × 10.sup.10                                                                  dynes/cm.sup.2                                    p.sub.y                     uC/m.sup.2 K                                      dielectric constant                                                                             12.2                                                        d.sub.33          16.5      pC/N                                              dp                2.7       pC/N                                              ______________________________________                                    

Example 3

One part by weight of Nylon 11 is dissolved in four parts of2-ethyl-hexane 1,3 diol at 150° C. The solution is transferred to a trayand placed in a vacuum oven. The oven is maintained at a vacuum of about10⁻³ torr and at a temperature of 50° C. until Nylon 11 solution isobtained having about 50% by weight of Nylon 11. The Nylon 11 solutionis transferred as a film to a press, subjected to a pressure of 1000 psiand heated to 200° C. The film is then rapidly cooled by immersion intoan ice bath. The Nylon 11 solution film now comprises 70% by weight ofNylon 11 and 30% by weight of 2-ethyl-hexane 1,3 diol. This film havinga thickness of about 1 mil is transferred to a poling apparatus of thetype depicted in FIG. 1 between two polished copper plates, is connectedto a high voltage DC supply, and then is placed under high vacuum (˜10⁻⁶torr). The Nylon 11 solution film is poled generally in a manner asshown in FIG. 2. The field is increased linearly from zero to 350 KV/cmin ten minutes, while maintaining the temperature at 22° C. Poling atthis field strength is continued for a further ten minutes at which timethe field is reduced to zero.

Example 4

One part by weight of Nylon 7 is dissolved in four parts of2-ethyl-hexane 1,3 diol at 170° C. The solution is transferred to a trayand placed into a vacuum oven. The oven is maintained at a vacuum ofabout 10⁻³ torr and at a temperature of 50° C. until Nylon 7 solution isobtained having about 50% by weight of Nylon 7. The Nylon 7 solution istransferred as a film to a press, subjected to a pressure of 1000 psiand heated to 220° C. The film is then rapidly cooled by immersion intoan ice bath. The Nylon 7 solution film now comprises 73% by weight ofNylon 7 and 27% by weight of 2-ethyl-hexane 1,3 diol. This film having athickness of about 1 mil is transferred to a poling apparatus of thetype depicted in FIG. 1 between two polished copper plates, is connectedto a high voltage DC supply, and then is placed under high vacuum (-10⁻⁶torr). The Nylon 7 solution film is poled generally in a manner as shownin FIG. 2. The field is increased linearly from zero to 350 KV/cm in tenminutes, while maintaining the temperature at 22° C. Poling at thisfield strength is continued for a further ten minutes at which time thefield is reduced to zero.

Example 5

Five parts of Kureha capacitor grade poly(vinylidene fluoride) (PVF₂)film are dissolved in 95 parts of tricresylphosphate at 185° C. Thesolution is transferred to a tray and placed into a vacuum oven. Theoven is maintained at a vacuum of about 10⁻³ torr and at a temperaturewithin the range of 150° C.-200° C. until a PVF₂ solution is obtainedhaving about 70 percent by weight of PVF₂ and 30 percent by weight oftricresylphosphate. The PVF₂ solution is transferred as a film to apress, subjected to a pressure of 3000 psi and heated to 185° C. Thefilm is then rapidly cooled by immersion into an ice bath. The cooledfilm has some crystallinity, approximately 12-13 percent The PVF₂ and23.5 percent by weight of tricresylphosphate. This film having athickness of about 1 mil is transferred to a poling apparatus of thetype depicted in FIG. 1 between two polished copper plates, is connectedto a high voltage DC supply. The PVF₂ solution film is heated to 160° C.The solution is poled as shown in FIG. 2 heated for about 60 minutes.During the poling, the temperature is decreased linearly at 2° C./min to30° C. and the poling field is increased linearly from 25 KV/cm to 1000KV/cm. When room temperature is reached, the field is reduced to zero.Retained solvent content is about 23.5 percent. The resulting product ispolarized.

Example 6

The process of Example 5 is repeated except that the solution of PVF₂ at160 ° C. is quenched below the Tg temperature to inhibitcrystallization. The quenched solution is poled using 1000 KV/cm whilethe temperature is increased linearly 1° C./min to room temperature atwhich point the poling field is reduced to zero. The retained solventcontent is about 23.5 percent. The resulting product is polarized.

The polarized product produced can be further processed by applying apoling field under vacuum following the general procedure of Example 1.

Example 7

Other materials such as other polymers and soluble ceramic materialsdescribed above can be used to provide polarized materials of thisinvention by following generally the procedures of Example 1 or 5 withany modifications within the skill of the art.

Example 8

The procedure of Example 4 is repeated using instead of Nylon 7, a blendcomprising 50 percent by weight of Nylon 7 and 50 percent by weight ofNylon 11. The product obtained is polarized.

What is claimed is :
 1. A process for preparing a polarized materialwhich process comprises the following steps:(1) forming a solution of amaterial capable of being polarized using a polarization solvent whichcan be removed by evaporation to provide a crystalline or amorphouspolarized material which is free or substantially free ofmechanically-induced orientation and has mechanical andelectromechanical properties isotropic in a plane perpendicular to thepoling field direction; (2) forming said solution into a desired shapewhich is adapted to poling; and (3) subjecting the shaped solution topoling using a poling temperature and an effective DC electric fieldhaving an intensity less than the level at which substantial dielectricbreakdown of the material occurs, until substantial polarization isattained.
 2. A process for preparing a polarized material which processcomprises the following steps:(1) forming a solution of a materialcapable of being polarized using a polarization solvent which can beremoved by evaporation to provide a crystalline or amorphous polarizedmaterial which is free or substantially free of mechanically-inducedorientation and has mechanical and electromechanical propertiesisotropic in a plane perpendicular to the poling field direction; (2)increasing the concentration of said material in said solution byremoval of a portion of said solvent so that said solution has asufficiently high viscosity to be formed and retained in a desiredshape; (3) forming said solution having a portion of said solventremoved into a desired shape which is adapted to poling; (4) subjectingthe shaped solution to poling, using a poling temperature and aneffective DC electric field having an intensity less than the level atwhich substantially dielectric breakdown of the material occurs, untilsubstantial polarization is attained.
 3. A process of claim 1 in whichthe material used is a polarizable amorphous material.
 4. A process ofclaim 3 in which an amorphous material is used which has a glasstransition temperature near or above which substantial degradationoccurs.
 5. A process of claim 4 in which the amorphous material used hasa glass transition temperature of at least about 200° C.
 6. A process ofclaim 4 in which the amorphous material used has a glass transitiontemperature in the range of about 200° C. to about 400° C.
 7. A processof claim 6 in which the amorphous material used is a poly(vinylidenecyanide/vinyl acetate) copolymer.
 8. A process of claim 1 in which thematerial used is poly(vinylidene fluoride).
 9. A process of claim 1 inwhich the material used is a vinylidene fluoride copolymer.
 10. Aprocess of claim 1 in which the material used is poly(vinylidenefluoride/trifluorethylene) copolymer.
 11. A process of claim 1 in whichthe material used is poly(vinylidene fluoride/tetrafluoroethylene)copolymer.
 12. A process of claim 1 in which the effective DC electricfield used is provided by corona discharge of by contact withelectrodes.
 13. A process of claim 12 in which the process iscontinuous.
 14. A process of claim 1 in which the material which ispolarized is a material which is a soluble ceramic material or a polymerhaving polymer units capable of being polarized, selected from the groupconsisting of vinyl units, vinylidene units, ethylene units, acrylateunits, methacrylate units, nylon units, carbonate units, acrylonitrileunits, cellulose units, units having fluoro, chloro, amide, ester,cyanide, carbonate, nitrile or ether groups, protein units ofcombinations thereof .